1. Technical Field
The principles of the disclosed invention are related to the testing of magnetically actuated switches formed on semiconductor substrates, and in particular, to the testing o such switches at the wafer level of manufacture, before singulation and packaging.
2. Description of the Related Art
A magnetic switch is an electrical switch that is activated by magnetic attraction or repulsion. FIG. 1A is a schematic view of a well known prior art magnetic switch 100 that includes first and second contact plates 102, 104 made from a ferromagnetic material such as, for example, nickel-iron. The first and second contact plates each lie parallel to an X axis, and are offset with respect to each other so that only their respective free ends 108, 110 overlap, and are spaced a small distance apart. Each of the first and second contact plates 102, 104 has a contact terminal 106, by which the switch 100 is coupled to an electrical circuit.
The magnetic switch 100 is a normally-open type switch that closes when exposed to a magnetic force of sufficient strength. FIG. 1B shows the switch 100 in proximity to a magnet 112, with the magnetic force of the magnet depicted as lines 114 that arc from the north pole to the south pole of the magnet. The magnetic north and south poles of the magnet 112 define a polar axis P of the magnet. The magnet 112 is shown positioned near the switch 100 with its polar axis P lying substantially parallel to the X axis, and thus also parallel to the first and second contact plates 102, 104. When the magnet 112 and the switch 100 are brought into close proximity, the ferromagnetic material of the first and second contact plates 102, 104 is exposed to the magnetic force, which induces a magnetic polarity in the first and second contact plates 102, 104 that is opposite the polarity of the magnet 112. Thus, when the magnet 112 has a north pole on the left and a south pole on the right, each of the first and second contact plates 102, 104 has a north pole on the right and a south pole on the left. Because of the relative positions of the first and second contact plates 102, 104, the left-hand end 108 of the first contact plate 102 is adjacent to the right-hand end 110 of the second contact plate 104. Under the influence of the magnet 112, the end 108 of the first contact plate 102 is polarized as a south pole, while the end 110 of the second contact plate 104 is polarized as a north pole. Accordingly, as the first and second contact plates 102, 104 flex slightly, the ends 108, 110 of the first and second contact plates are drawn together by magnetic attraction, thereby closing the switch 100.
If the magnet 112 is positioned so that its polar axis P is perpendicular to the X axis of the switch 100, as shown in FIG. 2A, the direction or polarity of the magnetic force will be balanced across the contact plates 102, 104, so that the contact plates will not become polarized as described with reference to FIG. 1A. Thus, in the position shown in FIG. 2A, the switch will be in the open position. Rotation of the magnet 112 around a Y axis that lies perpendicular to the X axis brings the magnet toward the parallel position shown in FIG. 1B. The Y axis is perpendicular to the page, and not shown, but can be understood from the axis markings on FIG. 1A. As the magnet 114 is rotated away from the perpendicular position, as shown in FIG. 2B, at some angle of rotation, sufficient polarity will be induced in the contact plates 102, 104 to cause the switch 100 to close.
FIGS. 3A and 3B show a normally-closed type magnetic switch 120 that includes first and second contact plates 122, 124, each having a contact terminal 126. The first and second contact plates 122, 124 lie parallel to each other and are substantially coextensive. As shown in FIG. 3A, ends 128, 130 of the first and second contact plates 122, 124 are in electrical contact with each other under normal conditions. FIG. 3B shows the magnetic switch 120 in an actuated condition. As described above with reference to the magnetic switch 100, when the first and second contact plates 122, 124 are exposed to the magnetic energy of a magnet oriented as shown in FIG. 1B, they become magnetically polarized in a similar fashion. However, because the first and second plates 122, 124 are coextensive, their respective north and south poles are directly opposite each other. The magnetic repulsion between the ends 128, 130 causes the first and second contact plates to flex away from each other, opening the switch 120.
Turning now to FIGS. 4 and 5, a magnetic switch 140 is shown, which is one of a large plurality of switches formed on a semiconductor wafer 142 using methods that are well known in the art. FIG. 4 shows a perspective view of a portion of the wafer 142, while FIG. 5 is a cross-sectional view of the switch 140, taken along lines 5-5 of FIG. 4. For the sake of clarity, it will be assumed that any magnetic switches discussed hereafter are positioned so that their longitudinal axes lie parallel to the X axis, and that the substrate surfaces on which they are positioned lie parallel to a plane defined by the X and Y axes, with the Z axis being perpendicular to that plane.
The switch 140 is one of a broad class of devices that are commonly referred to as microelectromechanical systems (MEMS) devices. The particular structure of the switch 140 is merely exemplary, inasmuch as there are a number of different configurations for MEMS type magnetic switches. The switch 140 includes a cavity 144 formed in the upper surface of the wafer 142, over which a dielectric layer 146 is formed. A conductive layer 148 is positioned over the dielectric layer 146 and a channel 150 is provided in the conductive layer 148 to electrically isolate the two sides of the switch 140. A first contact plate 152 of ferromagnetic material is positioned in the cavity 144, with a layer of conductive material 154 positioned on an upper surface thereof. A second ferromagnetic-material contact plate 156 is suspended over the surface of the substrate 142 by a pair of springs 158 extending from the second contact plate 156 to respective anchors 160 positioned on the surface of the substrate 142. Finally, a segment of a conductive layer 162 is positioned on an underside of an end 164 of the second contact plate 156, where it will touch the upper surface of the first contact plate 152 when the switch 140 is activated.
The ferromagnetic material of the first and second contact plates 152, 156 behaves substantially as described with reference to the first and second contact plates 102, 104 of FIGS. 1A and 1B. When the switch 140 is activated, the second contact plate 156 rotates around an axis defined by the springs 158 to bring its end 164 into contact with the upper surface of the first contact plate 152. The material of the conductive layers 148, 154, and 162 is selected to resist formation of oxides that could interfere with a good electrical contact upon closing, such as, e.g., gold. There may be as many as 6,000 to 8,000 switches formed on a single wafer.
During the manufacturing process, as shown in FIG. 6, following the formation of the switches on the semiconductor material wafer 142, a second wafer 170 is positioned above the first wafer 142 and bonded to the surface thereof, to form a composite wafer 171. The second wafer 170 includes a first plurality of cavities 172 in positions that correspond to each of the switches 140 so that each of the switches is hermetically sealed within an enclosed chamber. A second plurality of cavities 174 is formed in positions corresponding to contact terminals 176 on the first wafer 142. After the second wafer 170 is bonded to the first wafer 142, the second wafer is thinned, by removing a portion of the upper surface, at least far enough to open the second cavities 174, as indicated by dotted line T. Thereafter, the composite wafer 171 is cut into individual dice 180, which removes material between the kerf lines K of FIG. 6. Each die 180 now contains a single, hermetically sealed, magnetically operated switch, which is thereafter packaged according to requirements of a particular application.
In FIG. 7, the exemplary die 180 is mounted to a paddle 186 of a lead frame and electrically coupled via wire bonding to leads 184, all of which is encapsulated within a flat-pack type package 182. Following the packaging step, each switch 140 is tested for conformance to a specific set of performance parameters.